Not too impressed Rated 1 out of 5 stars
It doesn't allow you to validate pages protected by basic authentication using the web service. The downloadable pro tool isn't available for linux, I don't understand why you need one in the first place since you can do everything you need with XUL. Finally, it's not integrated at all with firefox it's basically a webservice which gives results in not a very user friendly manner. You are better off using W3C's since this addon doesn't give any added value.This review is for a previous version of the add-on (5.3.0).
Maybe you were expecting something else
The web service does validate pages protected by BASIC authentication (see http://www.totalvalidator.com/validator/ValidatorForm?type=url), and there is a Linux version of the advanced tool (see comment below). The W3C validator cannot check broken links, take screenshots, check accessibility, or spell check. More importantly only does limited HTML checks - eg it cannot check the values of attributes, and will not check firewall protected or local pages ...